The Watt-second rating (not Watt, but Watt-seconds) of a strobe refers to the amount of energy put into the head, and doesn't precisely correlate to the amount of light output, which depends on the efficiency of the head, and the reflector and/or diffuser being used. You can use Watt-seconds to compare relative output between heads of the same model, so through the same head 800 W-s will be one stop brighter than 400 W-s, but when comparing units of different manufacture, it's only a rough guide.